An appropriate fatigue test and definitive motor unit types have not been clearly delineated for the extraocular muscles. This needs to be done with precision in stably held single motor units. Since hysteresis exists in both extraocular and spinal cord innervated skeletal muscles the systems are unpredictable in that one stimulus frequency can produce many levels of force. We neither know what these hysteresis loops look like nor the degree of hysteresis in extraocular muscles. We also need to know if there are differences in hysteresis parameters that are dependent on motor unit type. We have chosen to study the cat inferior oblique muscle in an attempt to answer these questions. The inferior oblique muscle nerve has a long and accessible course through the orbit. The nerve can be teased to isolate single axons so that single motor units can be stimulated and the mechanical parameters of which contraction time, half decay time, fusion frequency, fatigue, and hysteresis can be studied along with their accompanying forces. A sensitive strain gauge will be used to measure the above properties with the inferior oblique muscle at isometric tension. A quadrupolar electrode will be used to stimulate the nerve and to accurately measure axonal conduction velocity. The cat eye movement system is similar to man's. In addition, it can be used to make valid comparisons to other motor unit studies, both extraocular and spinal. Our efforts to understand the basic mechanisms of motor control will be served and the disruptive effects of fatigue and drugs can be quantified in this precise movement system.